1. Field of the Invention (Technical Field)
The invention relates to a method for detection of rail defects, in particular rail top defects, in a railway-track by measuring an axle box acceleration signal of a rail vehicle.
2. Description of Related Art
Rail defects, in particular rail top defects, as referred to in this document are local short vertical geometrical deviations that may cause impact between the rails of the railway-track and the rolling wheels of a rail vehicle. Aspects like indentations, differential wear and differential plastic deformation, inhomogeneous rail material and a defective manufacturing process of the rails may contribute to this problem. Unless repaired a light rail top defect or squat will grow into a moderate defect, and subsequently into a severe defect. Rail fracture and damages to its fastening, the rail pads, sleepers and ballast may also ultimately occur if no remedial action is taken. From the point of view of railway operation, safety and availability, rail defects, in particular rail top defects, should be detected and removed at the earliest possible occasion in order to prevent their further development into more serious rail defects.
Most commonly rail defects, and squats in particular, are detected by human inspection or by an ultrasonic technique. For the human inspection inspectors walk along the rail to find the rail defects, or alternatively inspect photos or a video record of the rails. In any case the naked human eye is needed to carry out the inspection. The ultrasonic inspection technique is only applicable when the cracks are deeper than approximately 7 mm in order to allow that the ultrasonic technique can be used for reliable detection of such cracks.
It has also been proposed to use eddy-current technology for detection of rail top defects, and even the use of acoustic detection has been proposed, however this latter technique is only applicable for detection of severe rail top defects, which emit detectable impact noise.
In the article “A Measurement System for Quick Rail Inspection and Effective Track Maintenance Strategy” published in Mechanical Systems and Signal Processing 21 (2007), pages 1242-1254, by M. Boccilione, et al, instrumentation for measuring lateral and vertical axle box acceleration of a rail vehicle is proposed which is usable for detection of defects in a railway track.
The measured vertical axle box acceleration of a rail vehicle as is known from said article is usable for the detection of a severe rail top defect. The measured axle box accelerations at a rail top defect are basically vibrations stemming from three sources, being                1. Vertical vibrations of the track, including those of the rail, rail pads, fastening, sleepers, ballast etc.        2. Vertical deformation and relative motion of the wheel and rail at the defect, and        3. Vibration of the wheel set, including also those of the bearing and of the axle box.        
The above-mentioned vibration source number 2, being the vertical deformation and relative motion of the wheel and rail at the defect is the signal that is of interest. For severe rail defects, in particular rail top defects, the vibration sources 1 and 2 are relatively strong. These sources can however be distinguished because of their different frequency characteristics. For less severe rail defects, the vibration signals become less strong, and vibration source number 3 may become relatively more dominant than the other sources of vibration. Both aspects contribute to deterioration of the signal-to-noise ratio making it hard to detect light or moderate rail defects, in particular rail top defects.
EP-A-I 593 572 discloses a method for identifying locations along a track at which the wheel of a railway vehicle subjects the rail along which the vehicle is travelling to longitudinal forces, comprising the measuring of an acceleration signal of a wheel of the rail vehicle, wherein a longitudinal acceleration signal is used in combination and simultaneously with a vertical acceleration signal.